1. Field of the Invention
The present invention relates to a mixed gas-separating membrane module and a process for separating a gas fraction having a high membrane permeability from a mixed gas. More particularly, the present invention relates to a mixed gas-separating membrane module containing a plurality of hollow fibers which allows a specific gas fraction.contained in a mixed gas to permeate through the hollow fibers and to be removed from the mixed gas, and a non-permeated gas fraction of the mixed gas to be collected, as a process using the mixed gas-separating membrane module for separating the specific gas fraction from the mixed gas.
Still more particularly, the present invention relates to a mixed gas-separating membrane module appropriate for separating water vapor from an organic substance vapor contained in the mixed gas, and to a mixed gas separating process using the membrane module and appropriate for separating water vapor from an organic substance vapor contained in the mixed gas.
2. Description of the Related Art
As a dewatering method for an aqueous solution of an organic substance, Japanese Unexamined Patent Publication No. 63-267415 discloses a dewatering and concentration process for an aqueous organic substance solution, comprising preparing a mixed gas containing an organic substance vapor and water vapor by evaporating an aqueous solution containing an organic substance; bringing the mixed gas into contact with a primary side surface of a mixed gas-separating membrane made from an aromatic polyimide at a temperature of 70.degree. C. or more, to selectively allow the water vapor to permeate through the mixed gas-separating membrane and to be collected as a permeated gas fraction on the secondary side surface of the membrane; and collecting, as a non-permeated gas fraction, the organic substance containing vapor having a reduced water content at the primary side surface.
This Japanese publication discloses an example of the process as mentioned above, wherein the secondary side, on which the gas fraction (water vapor) permeated through the membrane is collected, is maintained under a high level of reduced pressure, to selectively allow the water vapor to permeate through the membrane and to be separated from the organic substance vapor-containing gas fraction. In another example of the above-mentioned process, although the secondary side of the mixed gas-separating membrane is not maintained under a reduced pressure, a dry gas flows, as a carrier gas, along the secondary side surface of the membrane, to promote the selective permeation of the water vapor through the membrane and the separation of the water vapor from the organic substance vapor.
The above-mentioned dewatering process for the mixed gas, wherein the removal of the water vapor is carried out while maintaining the secondary side of the mixed gas-separating membrane under a reduced pressure, or passing a carrier gas consisting of a dry gas along the secondary side surface of the membrane, has the following disadvantages.
The permeating rate of the water vapor through the mixed gas-separating membrane per unit area of the membrane is not always satisfactory, and thus the scale of the mixed gas-separating membrane module is enlarged.
Also, the dryness (dewatering degree) of the non-permeated gas fraction left on the primary side of the membrane cannot easily reach the desired high level.
Further, since the dewatering process is carried out under a high level of reduced pressure, a vacuum pump capable of generating a high level of vacuum, is necessary, and a large amount of energy must be consumed for driving the vacuum pump.
Furthermore, to obtain a high degree of dewatering of the mixed gas by using the carrier gas under the ambient atmospheric pressure, a dry gas which is expensive must be used as a carrier gas in a large amount.
To remove the above-mentioned disadvantages, Japanese Patent publication No. 2,743,346 discloses a process for dewatering a solution containing water and an organic substance, in which a solution containing water and an organic substance is evaporated to prepare a mixed gas comprising water vapor and an organic substance vapor;
the mixed gas is fed into a mixed gas-separating membrane module which contains aromatic polyimide mixed gas-separating membranes each having:
(a) a water vapor-permeating rate (P'H.sub.2 O) of 1.times.10.sup.-5 cm.sup.3 /cm.sup.2.multidot.sec.multidot.cmhg or more, and PA1 (b) a ratio (P'H.sub.2 O/P'org) of the water vapor-permeating rate (P'H.sub.2 O) to an organic substance vapor-permeating rate (P'org) is 100 or more, at a temperature of 70.degree. C. or more, to bring the mixed gas into contact with primary (feed) side surfaces of the membranes; the secondary (permeate) sides of the membranes are exposed to a reduced pressure of 50 to 500 mmHg; and an inert dry gas or a portion of a non-permeated gas delivered from the membrane module is passed, as a carrier gas, through the secondary sides of the membranes to thereby selectively allow the water vapor to permeate through the membrane from the primary (feed) sides to the secondary (permeate) sides thereof and to be separated from a non-permeated organic substance vapor-containing gas, and to collect the organic substance vapor-containing gas having a reduced water content. PA1 (1) a cylindrical container having a mixed gas-feed section having a mixed gas-feed inlet, a non-permeated gas-delivery section having a non-permeated gas-delivery outlet, and a middle section located between the mixed gas-feed section and the non-permeated gas-delivery section and having a carrier gas-feed inlet and a permeated gas-delivery outlet; PA1 (2) a bundle of a plurality of mixed gas-separating hollow fibers each comprising a shell portion and a hollow portion surrounded by the shell portion, each extending through the middle section of the cylindrical container and each having an end portion thereof opening to the mixed gas-feed section and an opposite end portion thereof opening to the non-permeated gas-delivery section; PA1 (3) a pair of a first hollow fiber-supporting disk which supports the end portions of the hollow fibers opening to the mixed gas-feed section and partitioning the middle section from the mixed gas-feed section, and a second hollow fiber-supporting disk which supports the opposite end portions of the hollow fibers opening to the non-permeated gas-delivery section and partitioning the middle section from the non-permeated gas-delivery section, the first and second hollow fiber-supporting disks supporting the hollow fibers in such a manner that the hollow fibers are spaced from each other, to leave a continuous space between the hollow fibers; and PA1 (4) a cylindrical film member surrounding the hollow fiber bundle to such an extent that the continuous space formed between the hollow fibers is connected to the permeated gas-delivery outlet of the middle section, PA1 feeding a mixed gas comprising a first gas fraction and a second gas fraction into the mixed gas-feed section of the cylindrical container through the mixed gas-feed inlet, to cause the fed mixed gas to flow through the hollow portions of the hollow fibers, the first gas fraction having a permeation rate through the shell portions of the hollow fibers in a ratio to that of the second gas fraction of 100 or more, thereby to allow the first gas fraction to permeate through the shell portions of the hollow fibers; PA1 simultaneously feeding a carrier gas into the middle section of the cylindrical container through the carrier gas-feed inlet, while reducing the pressure of the middle section of the cylindrical container, to cause the fed carrier gas to flow through the continuous space formed between the hollow fibers toward the permeated gas-delivery outlet, while forcibly diluting the permeated first gas fraction therewith; PA1 delivering the permeated first gas fraction diluted with the carrier gas through the permeated gas-delivery outlet; and PA1 collecting the non-permeated second gas fraction passed through the hollow portions of the hollow fibers, and received in the non-permeated gas-delivery section, through the non-permeated gas-delivery outlet.
The above-mentioned process and membrane module enables the water/organic substance mixed gas to be dewatered. However, there is a strong demand for a dewatering process and apparatus for the mixed gas with an enhanced efficiency. Also, there is a strong demand of a mixed gas-separating process and apparatus capable of separating a gas fraction having a high membrane-permeating property, which is not limited to water vapor, from a mixed gas with a high efficiency.
As a constitution of a conventional gas-separating membrane module, Japanese Examined Patent Publication No. 6-91,932 discloses a gas-separating module in which a specific fiber bundle assembly formed from a bundle of hollow fibers having a function of selectively allowing a specific gas fraction (for example, a hydrogen gas fraction) in various mixed gases to permeate through the hollow fibers, is contained in an appropriate arrangement in a cylindrical container having a material gas-introduction inlet, a permeated gas outlet and non-permeated gas-outlet, and the periphery of the fiber bundle is covered with a film member. However, the module disclosed in the above-mentioned publication is one to be applied to a recovery of hydrogen, and thus the permeation side of the separating membrane has a structure usable only for delivering the permeated gas. Therefore, this module cannot be used for the case where water vapor is separated from a mixed gas comprising an organic substance vapor and water vapor, by making the permeation side of the separating membrane into a reduced pressure condition and by flowing a carrier gas through the permeation side of the separating membrane.
Also, Japanese Examined Patent Publication No. 7-79,954 discloses a gas-separating membrane module having a bundle of a plurality of gas-separating hollow fibers. This module has a core pipe located in a substantially center portion of the hollow fiber bundle and having apertures connected to a non-permeated gas outlet, and a cylindrical partitioning plate arranged in the hollow fiber bundle along the hollow fibers. The purpose of the invention disclosed in the publication is to enlarge the gas flow path and to enhance the feed line speed of the material gas while maintaining the length of the hollow fiber-shaped separating membranes constant. However, since an empty space is formed between the outer side of the hollow fiber bundle and the module container, the gas passes through the empty space and thus does not effectively pass through the continuous space between the hollow fibers. Also, since, in certain portions of the module, the gas outside of the hollow fibers flows in the same direction as that of the gas within the hollow fibers, the gas-separation efficiency of this module is not sufficiently high in comparison with the countercurrent type module.